This invention relates to a low refractive flint glass type optical glass of a SiO.sub.2 --Nb.sub.2 O.sub.5 --R'.sub.2 O--F (where R' is an alkali metal element) having optical constants of a refractive index (Nd) of about 1.60 or below and an Abbe number (.nu.d) of about 57-40.
Known in the art are a variety of glasses having these optical constants. These glasses are of a relatively low refractive and low dispersion property in the property region of flint glasses and have been produced mostly as a SiO.sub.2 --PbO system glass. These glasses, however, have a serious drawback of containing a toxic PbO ingredient. For overcoming this drawback, the specification of U.S. Pat. No. 3,877,953 discloses a SiO.sub.2 --B.sub.2 O.sub.3 --Nb.sub.2 O.sub.5 --K.sub.2 O system glass and the specification of U.S. Pat. No. 4,719,186 disclose a SiO.sub.2 --B.sub.2 O.sub.3 --TiO.sub.2 --R'.sub.2 O system glass.
The proposed glasses having the above described optical constants, however, have the disadvantage that they are inferior in light transmissivity.
It is therefore an object of the present invention to eliminate the above described disadvantages of the prior art glasses and provide an optical glass which has optical constants of a refractive index (Nd) of about 1.60 or below and an Abbe number (.nu.d) of about 57 to 40 and has improved light transmissivity.